Why Does The PH Keep Dropping In My Nutrient Solution When Using RO Reverse Osmosis Water With Hydroponics or Aquaponics?

November 11th, 2011 by Craig Mullins

Hey y’all! We’ve been getting a number of questions about the nutrient solution of hydroponic setups, in particular, the dropping of PH with RO Reverse Osmosis water in new setups as well as old setups, and with a complete water and nutrient change.

There are a ton of reasons why the PH can keep dropping (eg. rotting roots, type of soilless media, too high nutrient PPM, etc.) but for the purpose of this article we are going to focus on RO water’s buffering ability.

I’m going to use the General Hydroponics 3 Part Advanced Nutrient System (FloraMicro, FloraGro and FloraBloom) as an example since it’s hugely popular among growers.

Most Hydroponic nutrient solutions are designed to work with well water or “city” water out of the box; not RO water. When you use reverse osmosis water you are basically taking everything out of it, causing the water to lose its ability to hold a stable PH.

How do you get the PH levels to go back up? Most growers might try adding some baking soda or General Hydroponics PH UP, but as many of you have discovered, it’s just a temporary solution; hours later the PH just drops like a rock.

So what’s the solution? You need to add some buffering compounds into the water. If you’re a fan of General Hydroponics, CALiMAGic is your answer. It’s a Calcium and Magnesium supplement derived from Calcium Carbonate, Magnesium Nitrate, and IRON DTPA.

The recommended amount depends on your water parameters and type of plants growing but generally you will be OK with 1 Teaspoon (5ml) per gallon. Confirm with a water test.

When I did a test of water in my city, my water started with a PPM of 180 and PH 7.9 then I ran it thru an RO filter. The PH stayed the same but brought PPM’s down to 10. I mixed up the General Hydro 3 part nutrients and it brought the PH to 5.6. Aerated the water for 30 minutes and checked PH again. It was the same.

I next checked the nutrient solution after 24 hours and the PH was 4.6. Used PH up again to set water to 5.6, waited 24 hours and same thing happened – ph returned to 4.6. I did this several more times over 7 days. The same results kept happening.

So starting over, I put in 1 teaspoon (5 ml) of General Hydroponics CALiMAGic per gallon of Reverse Osmosis water. It’s important to put this in first before any other nutrients. Then I put in the 3 parts of GH fertilizer (FloraMicro, FloraGro and FloraBloom); making sure to put in FloraMicro before the other 2 parts of FloraGro and FloraBloom.

This brought my PH to 5.6. I aerated the solution for 30 minutes checked PH again and it was the same, so I added a bit of PH up to bring solution to PH 5.8. Know that it may drop a tad over a couple days; mine dropped .1 the first day and held steady over the 14 day test period.

One thing to note is that I used the Mild Growth formula when doing these tests. The plants leaves were curling up and down and we’re getting burned. It looked like they were being over-fertilized but it was just because of nutrient lockout due to the low PH.

2 Responses to “Why Does The PH Keep Dropping In My Nutrient Solution When Using RO Reverse Osmosis Water With Hydroponics or Aquaponics?”

I have the same problem, unstable pH, but mine goes back up. It comes out of the tap at 8.4, I live in Arizona where the lime content of water is quite high. Using an ro system, the pH starts out at 6.4, but within 24 hours returns to 8.4. I’m using GH Flora. What can I use to keep the pH down.

I believe Advanced nutrients has the patent to technology that has eliminated the need for pH-PPM metering or even adjusting pH levels with their PPM Perfect technology. However their products aren’t cheap.